1. Field of the Invention
The present invention relates to a modulated supply stage, and particularly to such a stage in which a feedback loop is connected to provide an input to control selection of a low frequency switched supply. The feedback may be provided from the output of the low frequency switched supply stage or from the output of a high frequency correction stage.
2. Description of the Related Prior Art
It is known by those skilled in the art that envelope tracking (ET) and envelope elimination and restoration (EER) can give large improvements in efficiencies of power amplifier operation, particularly with signals such as orthogonal frequency-division multiplexing (OFDM) which have large crest factors. However, it is also known that the application of these techniques presents considerable difficulties due to the large powers and bandwidths involved. These difficulties become particularly formidable when applied to portable wireless terminals where the number of discrete components must be minimised and large dimensioned magnetics must be avoided.
An apparently simple solution would be to make the modulator a fast responding linear regulator. However, this would simply change power wasted in the power amplifier with that wasted in the linear regulator, resulting in no net gain in efficiency.
In order to gain some efficiency, some prior art implementations have been known to follow the switched mode supply with a low drop-out (LDO) fast responding linear regulator. This removes the errors inherent in the switched mode operation. However a problem arises in that there must be sufficient range in the linear regulator to allow for the peaks in the switched mode error, which can be considerably larger than the root mean square (RMS) error. This results in a large standing dissipation in the LDO.
A significant improvement on this is provided by techniques disclosed in GB2398648. This implementation is shown in FIG. 1. FIG. 1 shows a diagram of a typical switched mode power supply used as an efficient power conversion means. It must be noted that this is given as an example; the invention is not restricted to topologies of this type.
A coarse DC-DC switched supply 102 provides an approximation to the required waveform, provided as a reference waveform on input line 118, after filtering with filter network 104. The filter comprises an inductor 106 for storage of magnetic energy, and a capacitor 108 for storage of electric energy. A transformer 110 is used which can give true summation, e.g. signals can be added and subtracted, so the mean correction from a correction amplifier 114 can be set to zero, eliminating large standing dissipation. The output of the transformer provides an output to a load 112. The output of the transformer 110 is fedback to provide an input to the correction amplifier 114, which receives as a further input a reference signal on line 116 (which may be the same as, or derived from the same source as, the reference signal on line 118). The transformer combines the switched supply voltage with the output of the correction amplifier to provide a corrected output voltage.
A potential problem with the architecture of FIG. 1 is that the transformer 110 has to have a high self-inductance to prevent shunting of the correction current through the unwanted inductance of the transformer. This means that large ferrite cores must typically be used. Whilst this is acceptable for wireless infrastructure implementations, this presents particular difficulties for portable handset implementations or any implementation where size restrictions may apply.
The supply stage of FIG. 1 is capable of very efficient operation, but the circuit can only be switched between two levels: intermediate levels can only be obtained by the filtering action of the energy storage elements 106 and 108. For low frequency outputs (frequencies much less than the switching frequencies), this arrangement will be able to perform tracking, but the circuit may provide poor tracking at high frequency. There will also be substantial breakthrough of switching related products at high frequencies. When the said power conversion circuit is used as a modulator, the energy storage elements form a parallel resonant tank which will present a high impedance to the load at some frequencies.
The effect of this can be seen in FIG. 2. The reaction of the energy storage elements to the rapidly changing current demand produces a waveform 204 at the power amplifier. This shows severe mistracking when compared with the wanted waveform 202. Also, the high output impedance may result in instability of the load.
Examples of prior art switched mode modulators can be found in U.S. Pat. Nos. 5,905,407, 6,054,914, 6,198,374, 6,300,826, 6,583,664, 6,661,210, 6,661,217, 6,710,646, 6,792,252, and in US Patent application No. 2002/0008574.
It is an aim of the invention to provide an improved modulated power supply stage.